This invention pertains to microlithography (projection-transfer of a pattern, defined by a reticle or mask, onto a sensitive substrate such as a semiconductor wafer). Microlithography is a key technology used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to microlithography using a charged particle beam (electron beam or ion beam) as an energy beam. Even more specifically, the invention pertains to methods for making reticles as used in charged-particle-beam (CPB) microlithography, to reticles made using such methods, and to CPB microlithography methods performed using such reticles.
In recent years, as semiconductor integrated circuits have become increasingly miniaturized, the resolution limits of optical microlithography (i.e., projection-transfer of a pattern performed using ultraviolet light as an energy beam) have become increasingly apparent. As a result, considerable development effort currently is being expended to develop microlithography methods and apparatus that employ an alternative type of energy beam that offers prospects of better resolution than optical microlithography. For example, considerable effort has been directed to use of X-rays. However, a practical X-ray system has not yet been developed because of many technical problems with that technology. Another candidate microlithography technology utilizes a charged particle beam, such as an electron beam or ion beam, as an energy beam.
A current type of electron-beam pattern-transfer system is an electron-beam system that literally xe2x80x9cdrawsxe2x80x9d a pattern on a substrate using an electron beam. In such a system, no reticle is used. Rather, the pattern is drawn line-by-line. These systems can form intricate patterns having features sized at 0.1 xcexcm or less because, inter alia, the electron beam itself can be focused down to a spot diameter of several nanometers. However, with such systems, the more intricate the pattern, the more focused the electron beam must be in order to draw the pattern satisfactorily. Also, drawing a pattern line-by-line requires large amounts of time; consequently, this technology has very little utility in the mass production of semiconductor wafers where xe2x80x9cthroughputxe2x80x9d (number of wafers processed per unit time) is an important consideration.
In view of the shortcomings in electron-beam drawing systems and methods, charged-particle-beam (CPB) projection-microlithography systems have been proposed in which a reticle defining the desired pattern is irradiated with a charged particle beam. The portion of the beam passing through the irradiated region of the reticle is xe2x80x9creducedxe2x80x9d (demagnified) as the image carried by the beam is projected onto a corresponding region of a wafer or other suitable substrate using a projection lens.
The reticle is generally of two types. One type is a scattering-membrane reticle 21 as shown in FIG. 15(a), in which pattern features are defined by scattering bodies 24 formed on a membrane 22 that is relatively transmissive to the beam. A second type is a scattering-stencil reticle 31 as shown in FIG. 15(b), in which pattern features are defined by beam-transmissive through-holes 34 in a particle-scattering membrane 32. The membrane 32 normally is silicon with a thickness of approximately 2 xcexcm.
Because, from a practical standpoint, an entire reticle pattern cannot be projected simultaneously onto a substrate using a charged particle beam, conventional CPB microlithography reticles are divided or segmented into multiple xe2x80x9csubfieldsxe2x80x9d 22a, 32a each defining a respective portion of the overall pattern. The subfields 22a, 32a are separated from one another on the membrane 22, 32 by boundary regions 25, 35, in which no pattern elements are defined. In order to provide the membrane 22, 32 with sufficient mechanical strength and rigidity, support struts 23, 33 extend from the boundary regions 25, 35.
Each subfield 22a, 32a typically measures approximately 1-mm square. The subfields 22a, 32a are arrayed in columns and rows across the reticle 21, 31. For projection-exposure, the subfields 22a, 32a are illuminated in a step-wise or scanning manner by the charged particle beam (serving as an xe2x80x9cillumination beamxe2x80x9d). As the illumination beam passes through each subfield, the beam becomes xe2x80x9cpatternedxe2x80x9d according to the configuration of pattern elements in the subfield. As depicted in FIG. 15(c), the patterned beam propagates through a projection-optical system (not shown) to the sensitive substrate 27. (By xe2x80x9csensitivexe2x80x9d is meant that the substrate is coated on its upstream-facing surface with a material, termed a xe2x80x9cresist,xe2x80x9d that is imprintable with an image of the pattern as projected from the reticle.) The images of the subfields have respective locations on the substrate 27 in which the images are xe2x80x9cstitchedxe2x80x9d together (i.e., situated contiguously) in the proper order to form the entire pattern on the substrate.
Conventionally, reticles of the types summarized above are manufactured using semiconductor-fabrication technology. Fabrication begins with a silicon reticle substrate (typically having a thickness of 1 mm or less). The reticle membrane, subfields, and support struts are fabricated from the reticle substrate. The reticle conventionally is attached circumferentially to a peripheral frame typically having a thickness of about 10 mm. The peripheral frame, normally also made of silicon, strengthens the reticle for routine handling and during use of the reticle in the CPB projection-microlithography apparatus.
A conventional scattering-stencil reticle mounted to a peripheral frame is shown in FIGS. 16(a)-16(b). FIG. 16(a) depicts a reticle assembly 39 comprising a stencil-reticle portion 41 that includes a pattern-defining region 45 and a peripheral region 44. The pattern-defining region 45 includes multiple subfields 42 (each with a respective membrane portion) and support struts 43. The membrane portions have a thickness of about 2 xcexcm and define respective portions of the reticle pattern, as described above. If the stencil-reticle portion 41 has an outer diameter of about 8 inches, then the thickness of the peripheral region 44 is about 700 xcexcm. The edge region 46 of the stencil-reticle portion 41 is attached to a peripheral frame 40 having a thickness of about 10 mm.
Unfortunately, with reticles made by conventional technology, attachment of the stencil-reticle portion 41 to a peripheral frame 40 generates a stress throughout the stencil-reticle portion 41 that tends to cause warping (deformation) of the pattern-defining region 45. The warping extends to the subfields 42 and thus to the respective pattern portions defined by the subfields 42. This warping is especially a problem if the stencil-reticle portion 41 is attached to the peripheral frame 40 after the pattern has been formed on the pattern-defining region 45. The warping prevents attainment of sufficiently accurate pattern transfer.
Hence, there is a need for a reticle (for CPB microlithography) that is attached to a peripheral frame 40 but that exhibits substantially reduced warp in the pattern-defining region 45, compared to conventional reticles.
In view of the shortcomings of the prior art as summarized above, an object of the present invention is to provide reticles in which pattern warp is substantially reduced or reducible.
To such end and according to a first aspect of the invention, reticles are provided, for charged-particle-beam (CPB) microlithography, that comprise a reticle portion. In an embodiment, the reticle portion comprises a pattern-defining region, an inner supporting part, and an outer supporting part. The pattern-defining region comprises multiple subfields separated from one another by support struts. Each subfield defines a respective portion of a pattern defined by the reticle. The inner supporting part is attached peripherally to the pattern-defining region, and is configured to support the pattern-defining region integrally. The outer supporting part surrounds the inner supporting part and is connected to the inner supporting part by multiple connecting structures each having a spring characteristic. The outer supporting part is configured so as to support the inner supporting part and pattern-defining region in a peripheral manner. The reticle can further comprise a peripheral frame peripherally attached to the reticle portion. With such a reticle, stress triggered in the periphery of the reticle as a result, especially, of attaching a peripheral frame to the reticle is absorbed by deformation of the connecting structures rather than warping of the pattern-defining region.
The pattern-defining region can be configured as a stencil reticle in which pattern elements are defined as respective voids in a CPB-scattering reticle membrane. With such a reticle, the temperature of pattern-defining region does not increase excessively during use because the amount of charged-particle absorption by the pattern-defining region is relatively small, even with high illumination-beam currents. Thus, thermally induced warp is reduced. In any event, thermal warp and mechanically engendered warp are dissipated in the connecting structures.
Alternatively, the pattern-defining region can be configured as a scattering-membrane reticle in which pattern elements are defined as respective spaces between CPB-scattering bodies situated on a CPB-transmissive reticle membrane. Even with this type of reticle, temperature increase of the reticle during use is not excessive because the amount of absorption of charged particles by the reticle is small, even at high illumination-beam currents. In any event, thermal warp and mechanically engendered warp are dissipated in the connecting structures.
Each connecting structure can have an H-shaped configuration having two pairs of H-ends. In such a configuration, a first pair of H-ends is connected to the inner supporting part and a second pair of H-ends is connected to the outer supporting part. Alternatively, each connecting structure can have an X-shaped configuration having two pairs of X-ends. In this alternative configuration, a first pair of X-ends is connected to the inner supporting part and a second pair of X-ends is connected to the outer supporting part. With such structures, it is possible to define spring constants by matching the spring constant of connecting structure to a characteristic of mechanical strength (especially an elastic characteristic) of the reticle portion.
The reticle can comprise a number (n) of connecting structures each satisfying a relationship nKf=Ks/xcex2, wherein Ks is an in-plane elastomeric constant of the reticle portion, xcex2 is a connection-relaxation coefficient of the connecting structure, and Kf is a spring constant of the connecting structure. With such a configuration, if the number of connecting structures is excessive, then additional mechanical stress is imparted to the reticle portion, which is rendered easily warped. On the other hand, if the number of connecting structures is too low, then proper support of the reticle portion becomes too difficult to achieve. By satisfying this relationship, the reticle portion is supported adequately while inhibiting propagation of warp from the outer supporting part to the inner supporting part (and pattern-defining region).
According to another aspect of the invention, methods are provided for making a reticle for CPB microlithography. Inc an embodiment of such methods, a silicon-on-insulator (SOI) reticle substrate is provided. The reticle substrate comprises a base layer, a silicon oxide layer on an obverse surface of the base layer, and a silicon layer on the silicon oxide layer. An etching mask is applied to a reverse surface of the base layer. The etching mask defines respective openings at anticipated locations of reticle subfields in a patter-defining region. The etching mask also defines respective locations of an inner supporting part surrounding the pattern-defining region, an outer supporting part surrounding the inner supporting part, and multiple connecting structures connecting the inner supporting part to the outer supporting part. The base layer is etched anisotropically at openings in the etching mask. The etching is allowed to proceed depthwise through the base layer to the silicon oxide layer, so as to define the subfields, the inner supporting part, the outer supporting part, and the connecting structures. Afterward, the exposed regions of silicon oxide are removed. Desirably, each connecting structure is composed of silicon and is formed in the anisotropic etching step by selectively etching away complementary regions of the base layer by anisotropic etching. The connecting structures can be formed, in the anisotropic etching step, at the same time as supporting struts separating the subfields from each other in the pattern-defining region. By fabricating the connecting structures at the same time as the support struts, the time (and cost), required to fabricate the reticle is reduced.
The method summarized above can include the step of defining a chip pattern in the pattern-defining region, and/or the step of attaching a peripheral frame to the outer supporting part.
According to another aspect of the invention, CPB microlithography reticles are provided that are formed by any of the methods according to the invention.
According to another embodiment, CPB-microlithography reticles according to the invention comprise a reticle portion that comprises (1) a:pattern-defining region comprising multiple subfields separated from one another by support struts, wherein each subfield defines a respective portion of a pattern defined by the reticle; (2) an inner supporting part peripherally attached to the pattern-defining region and configured so as to integrally support the pattern-defining region; (3) an outer supporting part peripherally surrounding the inner supporting part; and (4) multiple connecting structures connecting the inner supporting part to the outer supporting part. Each connecting structure comprises a first conductive region situated on the outer supporting part and a second conductive region situated on the inner supporting part. At least the first conductive regions are selectively energizable electrically so as to cause, in a selective manner, the respective first and second conductive regions to move relative to each other, thereby displacing the pattern-defining region so as to cancel, at least partially, a warp of the patter-defining region.
In each connecting structure, the first and second conductive regions can exhibit an electrostatic attraction with respect to each other under appropriate conditions of electrical energization of at least the respective first conductive region.
The reticles can further comprise a peripheral frame peripherally attached to the outer supporting part. In such a configuration, the peripheral frame can comprise a conductive pad from which a wiring connection is made to a respective first conductive region.
Each of the first conductive regions can comprise a first flexible membrane member connected to the outer supporting part. Similarly, each of the second conductive regions can comprise a second flexible membrane member connected to the inner supporting part. In such a configuration, each connecting structure desirably further comprises an insulating member situated between the respective first and second flexible membrane members.
According to another aspect of the invention, CPB microlithography apparatus are provided. An embodiment of such an apparatus comprises an illumination-optical system, a projection-optical system, a reticle stage, and a substrate stage. The illumination-optical system is situated and configured to irradiate a charged-particle illumination beam onto a selected region of any of the various embodiments of a reticle, according to the invention, as summarized above. The reticle stage is situated and configured to: (i) hold the reticle as the reticle is being illuminated by the illumination beam, and (ii) selectively energize the conductive regions so as to reduce reticle warp. The projection-optical system is situated and configured to receive a patterned beam, formed by passage of the illumination beam through the reticle and carrying an image of the irradiated region of the reticle, and to focus the image onto a predetermined position on a sensitive substrate. The substrate stage is situated and configured to hold the substrate as the substrate is being exposed by the patterned beam.
According to yet another aspect of the invention, methods are provided for microlithographically exposing a pattern onto a sensitive substrate using a charged particle beam. In an embodiment of such a method, a reticle is provided that comprises: (i) a pattern-defining region comprising multiple subfields each defining a respective portion of a pattern defined by the reticle, (ii) an inner supporting part peripherally attached to the pattern-defining region and configured so as to support the pattern-defining region integrally, (iii) an outer supporting part peripherally surrounding the inner supporting part, and (iv) multiple connecting structures connecting the inner supporting part to the outer supporting part. Each connecting structure comprises a first conductive region situated on the outer supporting part and a second conductive region situated on the inner supporting part. At least one of the conductive regions is energized electrically in a selective manner so as to cause, in a selective manner, the respective first and second conductive regions to move relative to each other, thereby displacing the pattern-defining region so as to cancel, at least:partially, a warp of the pattern-defining region. The charged particle beam is irradiated selectively onto the subfields in an ordered manner to transfer the reticle pattern to the substrate.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.